Electro-optically active waveguide devices, such as modulators, have been prepared using lithium niobate (LiNbO3). Lithium niobate has a lattice structure in which the lithium ions have a non-centrosymmetric position. In the absence of an electric field, this non-centrosymmetric position imparts the material with a net polarization. Applying an electric field to the lithium niobate shifts the position of the lithium ions, changing the net polarization, and refractive index, of the material. Thus, the phase of light propagating through the material may be altered by applying an electric field to the material.
FIGS. 1A-1B schematically illustrate plan and cross-sectional views, respectively, of a previously known lithium niobate waveguide device 100 that includes lithium niobate substrate 101 in which waveguide 102 is formed, input optical fiber 111, output optical fibers 112, and voltage generator 121. Waveguide 102 may be formed by exchanging protons for some of the lithium ions in the substrate 101 within defined areas, e.g., by immersing the substrate into a solution containing a proton exchange acid, such as benzoic acid. The proton-exchanged areas have a higher extraordinary refractive index than the remainder of the substrate, and so act as a waveguide 102 that transports light through substrate 101 with relatively low loss. In the illustrated modulator 100, light is introduced to waveguide 102 through input optical fiber 111. Junction 105 of waveguide 102 divides the light into two portions and respectively guides the light portions into sections 106 and 107 of waveguide 102. Electrodes 122 are positioned on either side of the waveguide sections 106, 107, and separated from the waveguide sections by buffer regions 103. In one example, the inner edges of the electrodes are spaced approximately 10 microns from each other, with sections 106 or 107 therebetween.
Voltage generator 121 is programmed to independently apply voltages to different pairs of the electrodes 122, so as to change the phase of the light traveling through the waveguide section 106 or 107 adjacent that pair. For example, as illustrated in FIG. 1B, waveguide section 106 has a net polarization 104 in the absence of an electric field. Voltage generator 121 (not shown in FIG. 1B) may apply a voltage across electrodes 122, which generates an electric field along the crystallographic Z-axis to change the net electrical polarization 104 of waveguide section 106, which induces a phase change of light traveling through that section, and an equal change of polarization in waveguide 107 (not shown in FIG. 1B), but of opposite sign. The magnitude of the change in the material's net polarization and the magnitude of the phase change, may be proportional to the applied electric field. The light in waveguide sections 106, 107, may be coupled out of waveguide 102 and into separate output optical fibers 112, as illustrated in FIG. 1A.
Alternatively, in modulator 100′ illustrated in FIG. 1C, the light in sections 106′, 107′ of waveguide 102′ may instead be recombined at junction 108′, where they interfere with one another. Because the relative phase of the light portions traveling through waveguide sections 106′, 107′ may be controlled via voltage generator 121′, the intensity of the light at junction 108′ may be modulated as desired. For example, if the portion of light in section 106′ is phase delayed by an even multiple of π relative to that in section 107′, then the two portions of light will constructively interfere with each other, yielding maximum brightness. Or, for example, if the portion of light in section 106′ is phase delayed by an odd multiple of π relative to that in section 107′, then the two portions will completely interfere with each other, yielding minimal brightness. Any intensity in between may be selected by suitably adjusting the relative phase delays via voltage generator 121′. The output of waveguide 102′ is coupled into a single output optical fiber 112′. Configurations such as that illustrated in FIG. 1C may be referred to as a Mach-Zehnder modulator (MZM). Waveguide configurations other than those illustrated are also common.
One choice encountered in designing a lithium niobate-based modulator is the orientation of the crystal axes relative to the waveguides and electrodes, which affects the modulator efficiency. Specifically, lithium niobate primarily is used in one of two main crystallographic orientations: X-cut or Z-cut. The term X-cut or Z-cut refers to the surface-normal of the wafer being parallel to the X or Z crystallographic axes, respectively. The strongest component of the applied electric field preferably is aligned with the Z-axis of the crystal, which has the highest electro-optic coefficient. Whether an X-cut or Z-cut lithium niobate crystal is used, the electrodes preferably are arranged on either side of the waveguide. Because the ground electrodes reside on the same plane as the driving or bias electrodes, the applied electric field lines tend to be substantially parallel to the major planar surface of the device.
X-cut based modulators are particularly susceptible to charge accumulation on the surface of the crystal. For example, charge generation and charge redistribution may occur when a bias voltage is applied to an electrical input of a lithium niobate-based modulator. The bias voltage may cause the movement of mobile charges, in the form of electrons, holes, or ions, which may accumulate on the surface near the electrodes and either may counteract the effect of the applied voltage by establishing a positive drift in the bias voltage, or may enhance the applied bias voltage by establishing a negative drift in the bias voltage. Accordingly, the optimal bias voltage applied to the waveguide device may change with time. As a consequence, the outgoing light from the modulator also may deviate from the optimal output over time.
To compensate for drifts in the bias voltage, also known as DC drifts, and thus maintain the optimum bias voltage and optical output, it is common to employ voltage control circuitry. For example, such DC drifts may be electronically compensated for by monitoring the phase and/or intensity of the light output from the waveguide, and adjusting the magnitude of the electric fields to maintain the optical performance. However, the monitoring circuitry required to adjust the applied voltage may add significant expense and complexity to the modulator circuit, may limit the switching speed of the device, and may limit the environments in which the device may be used. Furthermore, as the applied DC bias steadily increases, the electric field between the device electrodes may damage the device materials during long-term operation, particularly at higher temperature. Such damage may further accelerate the DC drift and lead to further degradation in the optical performance of the modulator.
Methods are known in the art for reducing charge accumulation on the surfaces of lithium niobate devices. For example, U.S. Pat. No. 7,343,055 to McBrien discloses that some prior art devices may provide a metal oxide or semiconductor layer on top of the lithium niobate crystal and under the electrodes, so as to bleed off charges through a conductive path to the bottom of the device. McBrien further discloses that other devices may provide a conductive layer on the bottom of the device that is electrically connected with the ground electrodes to provide a discharge path. In addition, McBrien discloses that in such devices, charge accumulating on the signal electrode may find a path to ground through the driver or bias electronics. McBrien discloses addressing such a problem by arranging a first set of highly conductive RF electrodes as a transmission line on the top of the buffer layer, and maintaining a prescribed operating point by a second set of low conductivity electrodes in contact with the lithium niobate substrate. McBrien discloses that the electrodes located on the surface of the substrate, having an electrical connection to external terminals, can be used to eliminate substrate charging. McBrien further discloses that in Z-cut embodiments, the bias electrodes must be positioned above the optical waveguides to achieve the required electric field configuration, but that such an arrangement results in excessive optical loss. McBrien discloses that splitting the bias electrodes into two sections may reduce the optical loss, but results in a trade-off with modulation efficiency.
U.S. Pat. No. 7,231,101 to Nagata discloses use of a dielectric buffer layer to suppress absorption of propagating light that may be caused by a conductive overlayer. Nagata discloses that one of the plausible interactions between the charge bleed layer and the buffer layer is an enhancement of chemical defect generation at the interface of these layers, which may enhance electrical breakdown via the interface and cause the unwanted DC drift phenomenon.
Other methods for reducing charge accumulation are known. For example, U.S. Pat. Nos. 5,404,412 and 5,680,497, both to Seino, disclose reducing the effect of charge accumulation in a buffer layer by doping the buffer layer so as to enhance its conductivity. Nagata discloses the use of charge bleed-off layers and diffusion-inhibiting blocking layers to mitigate bias stability issues.
U.S. Pat. No. 5,214,724 to Seino discloses an optical waveguide device that includes a third electrode placed on the device surface, spaced from the pair of driving electrodes, and connected to a DC or low frequency voltage or to ground. Seino discloses that the third electrode concentrates the dispersed electric field in the vicinity of the signal electrode by causing a voltage drop that is proportional to the distance between the signal and the electrode and the third electrode.
What is needed is a lithium niobate waveguide device with improved stability.